Power conservation is important in battery powered devices such as pagers, computers, personal digital assistants, radiotelephones, cellular radiotelephones, and the like. These devices typically include a clock driver circuit. Power dissipation is of particular concern for the clock circuit because it is always active, even in standby mode.
It is known to use a standard CMOS output stage as a clock driver because of the favorable current characteristics. The current dissipation due to the load is: EQU Vcc*CL*F=2.75*10 pF*13 MHz=358 uA (Equation 1)
where Vcc is the supply voltage; CL is the load capacitance; and F is the frequency of the clock signal. However, the output waveform is almost a square-wave and it can produce significant harmonics at critical frequencies.
Harmonic distortion is a problem in communication devices if the reference clock has harmonic frequency components that exactly equal certain receive channel frequencies. This situation creates the potential for the reference clock to produce noise signals that are communicated via radiation and circuit board parasitics and interfere with the desired signal being received. The typical strategy for minimizing the harmonic distortion is to filter the output of the clock driver, making the output waveform closer to a sine wave than a square wave. With conventional circuits, this requires an off-chip filter, physically close to the driver. However this solution is expensive and the addition of a filter increases power dissipation.
The duty cycle of the clock signal is also an important consideration. The output voltage duty cycle is preferably no worse than 45-55%. This is particularly important where the clock signal drives a circuit using both clock edges, such as a digital speech processor integrated circuit (IC). If the duty cycle is worse than 45-55%, the communication device employing the clock signal may fail to operate.
Accordingly, it is desirable to provide a clock driver having improved current characteristics while maintaining the duty cycle and reducing harmonics.